The present invention relates to an improvement in a drive circuit for driving a piezoelectric (PZ) actuator comprised of a PZ stack.
A PZ actuator implemented by a PZ stack is often used with a dot matrix printer or an ink jet printer for driving a print head. A PZ stack generates, in response to an excitation voltage, a dimensional strain whose amplitude varies with time along a sinusoidal waveform. The dimensional strain is transmitted to a reciprocating mechanical object included in the print head, whereby a single dot is printed out. In the case of a dot matrix printer, the mechanical object is a print wire built in the print head while, in the case of an ink jet printer, it is a cell having a nozzle for jetting a drop of ink. The print head is constructed to print out one dot in response to every sinusoidal strain of the PZ stack. The PZ stack is so driven by a drive circuit as to generate adequate dimensional strains.
A prior art drive circuit includes a series connection of a PZ stack and a winding and applies an excitation voltage in the form of a rectangular pulse to the series connection every time one dot is to be printed out, as disclosed in U.S. Pat. No. 4,595,854. The PZ stack is charged by a resonance current which is caused to flow by the excitation voltage, whereby a dimensional strain proportional to the charge current is generated to cause a mechanical object into a reciprocating motion. A prerequisite with such a PZ stack is that the charge stored in the capacitance thereof be fully released before the application of the next printing pulse voltage. Otherwise, the residual charge would effect the intensity of a strain for printing out the next dot resulting in an uneven image density distribution. In the prior art drive circuit, however, the resonance current which flows through the PZ stack varies because the dot-by-dot strain of the PZ stack and the reaction of the mechanical object acting on the PZ stack are not constant, allowing residual charge to occur. It has been customary to provide the drive circuit with an exclusive discharging path so that after the application of an excitation voltage the residual charge may be dissipated through the resistance of the exclusive path. A problem with this approach is that the resistance consumes the current flowing during the charge dissipation as thermal energy, i.e., the residual charge is simply wasted to degrade the efficiency. Moreover, an extra period of time for discharging the residual charge is needed every time one dot is printed out, thus slowing down the printing operation. Another problem is that the winding has to have an inductance great enough to insure the resonance of the drive circuit, resulting in the need for a winding having a substantial size. For example, assuming a PZ stack made of ceramic whose proper oscillation frequency is about 3000 hertz, a resonance frequency of 3000 hertz which matches the proper oscillation frequency is needed. Since a PZ stack usually has a capacitance of about 0.3 microfarads, the resonance frequency of 3000 hertz mentioned above is not attainable without resorting to a winding having an inductance of 5 microhenries to 10 microhenries and, therefore, a large-size winding.